The present invention relates generally to angle-modulated signal transmission systems, More particularly, the invention relates to a system for transmitting a plurality of angle-modulated signals through respective transmission paths and having means for eliminating substantially all interference distortion due to crosstalk between the transmission paths.
In general, there is interference distortion responsive to crosstalk between a plurality of transmission paths for respectively transmitting angle-modulated signals. This interference distortion arises in the demodulated signals.
An example of a system having a plurality of transmission paths for transmitting the above mentioned angle-modulated signals is the 4-channel record disc recording and reproducing system previously described in U.S. Pat. Nos. 3,686,471 and 3,883,699. In the recording part of this 4-channel record disc recording and reproducing system, the signals of two pairs of channels are matrixed thereby to obtain respective sum and difference signals. A carrier wave is then angle-modulated with each difference signal. The resulting angle-modulated difference signal is multiplexed with a direct wave sum signal. The resulting multiplexed signal being recorded on a record disc by a cutter head.
In the reproducing system part of the 4-channel, the recorded multiplexed signal is reproduced from the record disc by a pickup cartridge. After the angle-modulated difference signals have been demodulated, the demodulated difference signals and the direct wave sum signals are used to recover the four original channel signals.
In this recording and reproducing system, crosstalk is apt to occur between the angle-modulated difference signals, particularly at the time of cutting and recording with the cutter head and at the time of reproducing with the pickup cartridge. With respect to this crosstalk due to the cutter head, by providing a crosstalk canceling circuit in the recording system. However, it has heretofore been difficult to cancel crosstalk due to the pickup cartridge because the characteristics of the pickup cartridges, used in the reproducing system, differ from cartridge to cartridge.
Heretofore, an attempt has been made to reduce the interference distortion arising from crosstalk in the reproducing system, by recording with a reduced level of signals in the low frequency band, which is angle-modulated with large modulation index. However, the low level of the low frequency range component has an important function with respect to so-called volume sensation in reproduced sound. The reproduced sound field becomes deficient in this sound volume sensation.
A study measures the relationship between (a) interference distortion and the phase difference between the two angle-modulated signals and (b) interference distortion and the amplitude ratios of the two angle-modulated signals. It was found that, irrespective of the characteristic of the pickup cartridge, each interference distortion becomes a minimum when the phase difference between the two angle-modulated signals is zero, and the amplitude ratio of the two angle-modulated signal is one (unity).
This result is indicated also by the following equations. By denoting two modulation signals by Ma and Mb, the crosstalk coefficient between angle-modulated signal transmission paths by K, the efficiency of angle-modulation and demodulation by .alpha., and the demodulation signal by E, the following equation is obtained. ##EQU1##
Here, the first term on the right-hand side is a demodulation signal with respect to the desired modulation signal. The second term is an interference distortion component created by crosstalk. When n = 1 in the second term, the interference distortion quantity Ed becomes: EQU Ed .apprxeq. .alpha. K sin (Mb - Ma)
It can be seen, therefore, that the distortion quantity Ed is very small, becoming zero in the ideal case, where the difference between the modulation signals (Mb - Ma) is very small, or ideally zero, that is, when the phases and amplitudes of the two modulation signals are respectively equal.